For all of the above fields of application, the prevailing means of performing imaging is with the use of radiation, usually X-rays, gamma-rays and beta-rays. Radiation is detected by some sort of imaging plane, which need not be planar. Accordingly, the term imaging surface will be used hereinafter. Images are formed either directly on the imaging surfaces (e.g. projection imaging) or data are processed and images are reconstructed in a computer (e.g. computerized tomography or coded aperture imaging in nuclear medicine).
The traditional imaging surface was formed by a film in a cassette. Other imaging surface solutions have been developed over the past 40 years offering digital imaging. Such examples are NaI scintillating screens, NaI scintillating crystals, BGO crystals, wire gas chambers, digital imaging plates etc. More recently, semiconductor imaging solutions such as Charged Coupled Devices, Si microstrip detectors and semiconductor pixel detectors have been developed.
Typically, in all of the above cases, when a large imaging area is needed it is made either as a monolithic structure (e.g. films, digital imaging plates, NaI screens etc.) or as a mosaic of smaller pieces (tiles) put together and fixed on a support surface (e.g. gamma cameras with NaI crystals).
When a monolithic large imaging surface is employed, if a part of the surface is defective then the whole surface needs to be changed. Unfortunately, the most precise digital on-line imaging devices proposed so far involve pixel-based semiconductors which cannot be manufactured in large areas (larger than a few square cm at most). Moreover, it would not be desirable to manufacture, for example, a monolithic 30 cm by 30 cm digital imaging semiconductor surface because the yield would be low. If a portion of the expensive imaging area became defective, then the whole surface would have to be replaced.
It has been proposed to provide a large area imaging surface (larger than a few square mm) using a tiling approach. In a recent patent application (EP 0 577 487 A1) semiconductor pixel detectors are tiled together to form a larger fixed surface. Using such an approach, individual imaging devices are arranged in a mosaic on an imaging support to form an imaging mosaic. Outputs from the individual imaging devices can be processed to provide a single output image corresponding substantially to the whole area covered by the imaging surface. The imaging devices are fixed to the imaging support in an essentially permanent manner by soldering or the like so that it is difficult or impossible to remove the imaging devices without damage to the devices and/or the support and/or the devices surrounding the device to be removed.
In a construction shown in U.S. Pat. No. 5,391,881, a large imaging device is made by several detectors joined side by side and arranged in the form of a matrix, the detectors being interconnected and associated with several integrated circuit chips arranged in matrix form behind the detectors for reading the signals from the detectors' elements. The integrated circuit chips are displaced relative to the detectors so as to partially overlap two or more detectors. The detectors are connected to the integrated circuit chips in an essentially permanent manner by microspheres, and the overlapping of the chips produces a fixed tiled structure.
It is seen from the above that solutions to large imaging areas proposed so far rely either on monolithic structures or on fixed tiling solutions. As high resolution semiconductor imaging solutions are progressively introduced in radiation imaging there is a need for an efficient way of assembling large imaging areas. Semiconductor imaging solutions are quite expensive both because of the semiconductor cost and also because of the integrated readout electronics. Therefore it is important to maximize the effective use of each square unit of imaging area.
Reference may be made to GB-A-2030422 which describes an x-ray tomography system with replaceable detector modules which are slidably received in radial compartments in an arcuate frame. However, each detector is quite narrow and is spaced from adjacent detectors, making this system unsuitable for large area "tiled" image detectors.
In a different field, U.S. Pat. No. 4,891,522 describes a 3-dimensional multi-layer high energy particle detector for a calorimeter. Each layer consists of a number of detector modules removably secured to a carrier member by conductive adhesive tape. An essential part of that design is that each detector module includes a feed-through connection pin for electrical connection to a corresponding detector module in an adjacent layer. Such a 3-dimensional calorimetric detector does not relate to the field of imaging, and does not address the problems in providing a tiled image detector surface or plane. Further, the resolution is limited to the size of each detector module, which means that the design is not suitable for high resolution pixel imaging.